A technique for driving and controlling an AC motor using an inverter has been widely used in the industrial field. The technique is conventionally and widely used also in electrical railroads. However, it is known that in a case of applying the above system to a DC feeding electrical railroad, electrical oscillation occurs in a harmonic-absorption LC filter circuit arranged on the DC side of an inverter and constituted by a reactor and a capacitor, and a voltage across the capacitor (a capacitor voltage) oscillates, which makes a control over the motor unstable. A damping control method for suppressing this oscillation is disclosed in Patent Literature 1 mentioned below.
Patent Literature 1 mentioned below discloses a configuration that controls an inverter to have a positive resistance characteristic with respect to a fluctuation amount of a capacitor voltage Efc by configuring the control apparatus such that a torque command Tm* is multiplied by a damping manipulated variable that is a value obtained by squaring a fluctuation rate of the capacitor voltage Efc and suppresses the electrical oscillation of an LC filter circuit, thereby making a control over the motor stable.